The disclosure of Japanese Patent Application No. 2001-347718 field on Nov. 13, 2001 including the specification, drawings and abstract are incorporated herein by reference in their entirety.
1. Field of the Invention
The invention relates to an exhaust emission control apparatus of an internal combustion engine, and control method of the same.
2. Description of the Related Art
Currently, an adsorption reduction type NOx catalyst is widely known as a catalyst. The adsorption reduction type NOx catalyst absorbs NOx in exhaust gas when an air-fuel ratio of exhaust gas flowing thereinto is lean, as the NOx catalyst which is to purify nitrogen oxides (NOx) in exhaust gas emitted from an internal combustion engine. Also, the adsorption reduction type NOx catalyst releases the absorbed NOx when the air-fuel ratio of exhaust gas flowing thereinto becomes rich. Then, it reduces and purifies the released NOx by hydrocarbons (HC) in exhaust gas.
The above-mentioned adsorption reduction type NOx catalyst is mainly used in most of the engine operating ranges, in order to purify the NOx in exhaust gas emitted from the internal combustion engine in which a combustion is executed at a lean air-fuel ratio. When a combustion is executed at a lean air-fuel ratio in most of the engine operating ranges, the air-fuel ratio of exhaust gas flowing into the NOx catalyst is also lean. Therefore, the NOx catalyst continues absorbing NOx for the duration of time that the air-fuel ratio of the inflow exhaust gas is lean.
Meanwhile, there is a limit to the amount of NOx that can be absorbed in the NOx catalyst. Once the amount of NOx absorbed by the NOx catalyst reaches the limit value, the NOx catalyst can no longer absorb NOx. Therefore, after that, NOx, which has flowed into the NOx catalyst, is not absorbed in the NOx catalyst and flows out of the NOx catalyst, which results in deterioration of exhaust emission as a whole.
In order to suppress deterioration of exhaust emission due to such a reason, it is necessary to detect that the total amount of NOx absorbed in the NOx catalyst (hereinafter referred to as a total NOx absorption amount) has reached the limit value, or that the total NOx absorption amount has come close to the limit value. Then, before NOx flows out of the NOx catalyst downstream, it is required to supply the NOx catalyst with exhaust gas with a rich air-fuel ratio, release NOx from the NOx catalyst, and reduce and purify the released NOx.
Accordingly, Japanese Patent Laid-Open Publication No.7-166851 discloses a NOx sensor for detecting a NOx concentration in exhaust gas flowing out of the NOx catalyst, is provided downstream of the NOx catalyst. This is in order to supply the NOx catalyst with exhaust gas with a rich air-fuel ratio, when the NOx concentration in exhaust gas flowing out of the NOx catalyst exceeds a predetermined concentration. More specifically, when the total amount of NOx absorbed in the NOx catalyst (total NOx absorption amount) has come close to the limit value, the ratio of NOx absorbed in the NOx catalyst among the NOx in exhaust gas drops, which causes the NOx concentration in exhaust gas flowing out of the NOx catalyst to increase gradually.
In the above-mentioned publication, it is determined on the basis of such a phenomenon that the total NOx absorption amount is close to the limit value when the NOx concentration in exhaust gas flowing out of the NOx catalyst exceeds the predetermined concentration. Then, exhaust gas with a rich air-fuel ratio are supplied to the NOx catalyst, NOx is released from the NOx catalyst, and the released NOx is reduced and purified by HC in exhaust gas, which suppresses deterioration of exhaust emission as a whole.
Meanwhile, a NOx concentration in exhaust gas flowing out of the NOx catalyst sometimes increases instantaneously depending on exhaust gas flow rate, etc. In the above-mentioned publication, the timing for supplying the NOx catalyst with exhaust gas with a rich air-fuel ratio is determined based on a NOx concentration. The NOx concentration is calculated only based on data obtained from the NOx sensor. Namely, the NOx concentration calculated in the above-mentioned publication is the NOx concentration in exhaust gas at a certain instant. Therefore, it is determined that the total NOx absorption amount is close to the limit value even when the NOx concentration in exhaust gas flowing out of the NOx catalyst has instantaneously increased. This means that exhaust gas with a rich air-fuel ratio are supplied to the NOx catalyst excessive number of times, which results in poor fuel economy as a whole.
Therefore, it is an object of the invention to estimate a total NOx amount absorbed in the NOx catalyst based on a NOx concentration in exhaust gas flowing out of the NOx catalyst more precisely.
In order to achieve the object, an exhaust emission control apparatus of the internal combustion engine according to an aspect of the invention includes, a NOx catalyst which absorbs NOx in exhaust gas when an air-fuel ratio of exhaust gas emitted from the internal combustion engine is lean, and also releases the absorbed NOx to reduce and purify the released NOx by hydrocarbon in the exhaust gas when an air-fuel ratio of exhaust gas is rich; a NOx sensor which detects a NOx concentration in exhaust gas flowing out of the NOx catalyst; a control portion which calculates a total NOx amount flowing out of the NOx catalyst while the vehicle runs for a predetermined distance based on a NOx concentration detected by the NOx sensor, calculates a NOx outflow amount per unit running distance based on the total NOx outflow amount, and supplies the NOx catalyst with exhaust gas with a rich air-fuel ratio when a NOx outflow amount per unit running distance exceeds a predetermined amount. Namely, according to this, a total NOx amount absorbed in the NOx catalyst is estimated based on an average NOx outflow amount obtained while the vehicle runs for a predetermined distance.
Also, a total NOx amount absorbed in the NOx catalyst can be estimated, in a similar manner, based on a total NOx outflow amount per unit time of the internal combustion engine or per unit engine speed.